FIMBRIATION AND COLONY TYPE OF <i>MORAXELLA BOVIS</i> IN RELATION TO CONJUNCTIVAL COLONIZATION AND DEVELOPMENT OF KERATOCONJUNCTIVITIS IN CATTLE

Abstract

Morphological variants of four Moraxella bovis strains were studied, using different cultural conditions. On bovine blood agar medium two colony types could be distinguished: 1) large, flat, rough and agar‐corroding, and 2) most often smaller, convex, smooth and non‐corroding forms. When transferred to human blood agar, the flat type formed convex, spreading‐corroding colonies (SC type) and the other type remained convex, non‐spreading and non‐corroding (N type). Some cell lines lost the original flat colony appearance on bovine blood agar after passages for a longer period on human blood agar; on bovine blood agar these SC cells now formed convex, but still corroding colonies. By electron microscopy it was observed that cells from corroding colonies on either medium were strongly fimbriated, whereas preparations from non‐corroding populations had non‐fimbriated cells or contained only an occasional fimbria. Conjunctival infection experiments in calves revealed that only fimbriated variants of each strain were able to colonize the conjunctival mucosa. Ultraviolet irradiation of the eye prior to inoculation facilitated the bacterial colonization, with respect to initiation and duration. Non‐fimbriated M. bovis could not be reisolated even after irradiation. Keratitis and conjunctivitis only occurred in irradiated eyes inoculated with cell lines yielding typically flat, corroding colonies on bovine blood agar. The possible function of fimbriae for attachment of M. bovis to the mucous membranes of the eye is discussed.